1.7. 3D Structure

Dashed-wedged line structure

A widely used way of showing the 3D structure of molecules is based on the skeletal formula, but using of dashes, wedges, and straight lines. This drawing method is essential because the placement of different atoms could yield different molecules even if the molecular formulas were exactly the same. Below are two drawings of a 4-carbon molecule with two chlorines and two bromines attached.

stick1.jpg stick2.jpg
4-carbon molecule with 2 chlorines and 2 bromines 4-carbon molecule with 2 chlorines and 2 bromines

Both drawings look like they represent the same molecule; however, if we add dashes and wedged bonds we will see that two different molecules could be depicted:

dashwedge1.jpg

The two molecules above are different, prove this to yourself by building a model. An easier way to compare the two molecules is to rotate one of the bonds (here, it is the bond on the right):

dashwedge2 (1).jpg

Notice how the molecule on the right has both bromines on the same side and chlorines on the same side, whereas the first molecule is different. Read about dashed-wedged Line structures, bottom of page, to understand what has been introduced above. You will learn more about the importance of atomic connectivity in molecules as you continue on to learn about stereochemistry.

The following are examples of each, and how they can be used together.

DWL.jpg

dashed.jpg

Blue bead= OH group; White bead=H

Above are 4-carbon chains with attached OH groups or Cl and Br atoms. Remember that each line represents a bond and that the carbons and hydrogens have been omitted. When you look at or draw these structures, the straight lines illustrate atoms and bonds that are in the same plane, the plane of the paper (in this case, computer screen). Dashed lines show atoms and bonds that go into the page, behind the plane, away from you. In the above example, the OH group is going into the plane, while at the same time a hydrogen comes out (wedged).

Wedged lines illustrate bonds and atoms that come out of the page, in front of the plane, toward you. In the 2D diagram above, the OH group is coming out of the plane of the paper, while a hydrogen goes in (dashed).

Wedged.jpg

Blue bead= OH group; White bead=H

As stated before, straight lines illustrate atoms and bonds that are in the same plane as the paper, but in the 2D example, the straight line bond for OH means that it it unsure or irrelevant whether OH is going away or toward you. It is also assumed that hydrogen is also connected to the same carbon that OH is on.

line.jpg

Blue bead= OH group; H is not shown

Try using your model kit to see that the OH group cannot lie in the same plane at the carbon chain (don’t forget your hydrogens!). In the final 2Dexample, both dashed and wedged lines are used because the attached atoms are not hydrogens (although dashed and wedged lines can be used for hydrogens).The chlorine is coming out the page while bromine is going into the page.

CLBR.jpg

Blue bead=Cl; Red bead=Br